No doubt, Ethernet and the TCP/IP Protocol are achieving more and more acceptance in the industrial automation technology. Major technical advances such as Fast Ethernet, Switching and Full Duplex communication have turned the good old Ethernet into a powerful communication system with a magical attraction to industrial users and manufacturers. Dreams of a standard application protocol for industrial automation seem highly unlikely. The same question that was asked when Fieldbus technology was introduced, "Will there be one standard?" has again being asked about Industrial Ethernet. The answer seems to be a definate NO!. There are currently 14 different Industrial Ethernet protocols on offer.
does Ethernet offer?
Intergration to the office world, IT-functions, Internet/Intranet, remote configuration. This is basically TCP/IP on Ethernet with application protocols such as SNMP, FTP, MIME, HTTP. Communications over routers and servers where IP-addressing and TCP transport are mandatory.
Greater bandwidth and larger data packages for communications with more and more intelligent industrial devices.
Quicker Real-Time communication with synchronisation good enough even for demanding motion control applications.
Connecting and addressing more devices over wider areas.
Homogenous networking mainly using Ethernet
Manufacturing Execution Systems (MES), on line updating of firmware and remote configuration and error handling.
Despite the different approaches in layer 7, all concepts have a common core. This common functionality includes the well established standards for Layer 1-4 such as Ethernet IEEE 802.xx data transmission technology (Layer 1), the bus access method (CSMA/CD, Layer 2), the Internet Protocol (IP, Layer 3) and the TCP and UDP protocols (Layer 4). In addition common elements can be found in Layer 7 for non time-critical functions. Here, international accepted IT-standard such as the Hypertext Transfer Protocol (http), the File Transfer Protocol (FTP) and the SNMP Simple Network Management protocol will be commonly used.
The differences between the current approaches can be found in the general communication system architecture, the industrial application protocols in layer 7, the object modeling and the engineering model for system configuration. The different concepts can be subdivided into encapsulation systems such as EtherNet/IP, Foundation Fieldbus HSE, Modbus-TCP distributed automation concepts such as Profinet.
The term encapsulation is used to describe the packaging (or embedding) of a telegram frame into a TCP or UDP container. A typical example of this approach is EtherNet/IP, which was developed by Rockwell Automation and the ODVA, the High Speed Ethernet (HSE) technology from Fieldbus Foundation and Modbus-TCP/IP. With all of these concepts, a more or less unchanged Fieldbus telegram is embedded as "user data" in a TCP/UDP frame before sending it over Ethernet. The advantage of this method is that the benefits of Ethernet as powerful, scalable communication media can be perfectly combined with the related existing Fieldbus solution without the need to change the overall communication philosophy or the engineering tools. Another advantage is, that the completion of the specifications did not require long developments. Accordingly, the first commercial products are already available and in industrial use. It is easy to provide downwards compatibility to the respective Fieldbus protocols on which each system is based. In these concept, Ethernet is mainly considered as a new data transmission technology which can be used as an alternative to – or in good combination with – the already established conventional fieldbusses such as DeviceNet, ControlNet, Modbus or Foundation Fieldbus H1.
Systems for Distributed Automation
The second category of Ethernet based industrial networks is aimed to fulfill the communication requirements of new automation concepts with distributed intelligence. In this approach, the overall application is distributed on several decentralized controllers which are connected via Industrial Ethernet. Profinet realizes only the non-time critical control functions via Ethernet and includes a gateway concept to interconnect with the existing Profibus technology for time critical real time communication.
Real Time vs IT functions
Ethernet communication with TCP/IP is non-deterministic and the reaction time is often above 100ms. Remote I/O demands reaction in the 5-10ms region. Motion Control craves even higher demands determinism with cycle times into the microsecond region. The conflict between IT-traffic using TCP/IP and the achievement of realtime performance is dealt with in several different ways.
Embedded Fieldbus or application protocol
By using standard TCP/IP embedding a fieldbus protocol at the application level maintains full IT-openness. You just tunnel the fieldbus protocol on Ethernet. Reaction time is around 100ms like standard Ethernet. In local segments with few devices and small data packages the reaction time goes down to 20ms. By using UDP instead of TCP the reaction time comes down to 10ms at best. Direct MAC-addressing in a local segment can bring reduce this closer to 1ms. Time synchronisation can be added such as that obtained by applying IEC61588. Bandwidth for TCP/IP traffic remains at 90-100%.
Closed Ethernet segment and
special DLL-layer for the realtime devices
Realtime is achieved by special protocol, on Layer 2 in the OSI-model, in every device in the realtime segment. For fast realtime cycling down to 0.2ms the segment is closed for IT-traffic. The realtime cycle is divided into slots where one slot is open for normal TCP/IP traffic. With 0.2ms cycle time and eight devices in the segment the slot time is about 22µs which allows for only small TCP/IP messages (up to 200 bytes). Bandwidth available for TCP/IP is approximately 1%.
Application protocol on TCP/IP,
direct MAC-addressing with prioritised messages
for realtime and hardware switching for fast
Standard TCP/IP messaging in parallel with IT-traffic is used for everything that doesn't need realtime handling. A parallel [virtual] channel is created by according higher priority and direct MAC addressing on packets for realtime operation down to 1ms. A third channel is created for fast realtime down to 0.2ms using a built-in switch in those devices that need fast realtime. These switches reserve one part of the cycle for realtime traffic but leave the major part open for normal TCP/IP IT traffic. Bandwidth for TCP/IP 50-100%.
Realtime on TCP/IP achieved
by prioritised messages and time synchronisation
Solutions aimed at motion control in parallel with standard IT traffic running on TCP/IP. The realtime operation is achieved through prioritised messages with a time stamp understood by the synchronised devices. The time latency in switches (up to a few hundred microseconds) is compensated within the devices. Realtime down to about 1ms with a jitter of 10µs. Bandwidth for TCP/IP 90-100%.
Realtime achieved on Ethernet
physics with built-in electronics and special
protocol from OSI Level 2 upwards
Special messages run on Ethernet physics in a ring or on double cables. The realtime is achieved through builtin electronics (e.g. asic) plus special addressing and protocol. TCP/IP with small messages can be embedded in the special protocol message and then unpacked in a master for forwarding onto the normal Ethernet. Connection of normal Ethernet devices uses hardware gateways. Bandwidth for TCP/IP is about 1%.
Conclusion from a device manufacturers view
Device manufacturers, who want to be successful on the international markets will have to implement all the different system approaches into their devices. Even if there are common functions in Layer 1 – 4, the application protocols and system models are totally different. Implementing the different standards into all the devices, will require a huge amount of development resources.
The same questions first asked about Fieldbuses 10 years ago are being asked now. There will be no standard Ethernet protocol and your product would probably need to support at least one Industrial Ethernet protocol. So which one do i develop? The answer is not only a performace based solution, but also a demographic question. Where do i want to sell my product? Modbus-TCP has the most installed nodes to date globally and was one of the first Ethernet protocols out on the market. For the USA EtherNet/IP is already well established and supported heavilly by Rockwell Automation and is catching Modbus-TCP at a great pace. For Europe then you also need to add Profinet to this. Recently released and backed by Siemens it will probably be the most important Ethernet protocol that your product might need to support. What about asia, what Ethernet protocols are popular there. This questions again go on and on and there is no definate right answer on which Ethernet protocol to support.
Conclusion from an End users view
Compared to the status of the situation with the conventional Fieldbus systems, users have good reasons to be optimistic about the future of Industrial Ethernet. Simply the fact that all upcoming Ethernet-based industrial networks all use the same data transmission technology is a considerable advantage in comparison to the large number of different transmission technologies of the fieldbusses. On top of the unique Ethernet physical layer, there will be several different and non-interoperable Industrial Ethernet application protocols.
From today's view, Industrial Ethernet will not replace the conventional
fieldbusses. Instead it will open new applications and will support
the migration towards distributed intelligence in automation.